Electronic fuel injectors are frequently used in today's internal combustion engines. The electronic fuel injector provides precise and reliable fuel delivery into the cylinder of compression ignition and spark ignition engines. The precision and reliability of the electronic fuel injector have contributed to the goals of fuel efficiency, maximum practicable power output and control of undesirable products of combustion. These and other benefits of electronic fuel injection systems are well known and are appropriately used to beneficial effect in the design of modern internal combustion engines.
Known electronic fuel injectors, especially those designed for application in spark ignition or compression ignition engines, utilize mechanical means to enhance fuel charge pressurization. Enhanced fuel charge pressurization is desirable during the fuel injection event to assure proper atomization and spray distribution of the fuel into the engine cylinder or pre-chamber. In addition, it is desirable to be able to determine the quantity of fuel used and to control the injection timing for several reasons, including obtaining full combustion of the fuel to control particulate emissions. This has been of great interest in recent years, owing to environment concerns and regulatory incentives. Finally, the proper control of fuel injectors reduces the amount of residual particulate formed in the compression ignition engine cylinder.
Several known types of fuel injectors include a means for the mechanical pressurization of the fuel charge. These fuel injectors have mechanical linkage systems coupled to the engine camshaft and/or cylinder head valve train assembly. Such fuel injectors are configured so that the camshaft or other rotating or reciprocating member acts on an injector link or equivalent structure either directly or indirectly through a rocker arm.
The injector link is generally vertically oriented with respect to the injector. Typically, displacement of the link in the downward direction (along the vertical axis) also causes an injector coupling to move downward within a bore created in the injector body. The coupling is spring-loaded and is returned to its original position by the force of a coupling return spring. The injector coupling is attached to a timing plunger and movement of the coupling causes relative movement of the timing plunger. When the injector coupling moves downward, the timing plunger moves downward into a timing plunger chamber. The timing plunger chamber, filled with fuel at the original fuel rail pressure of 150 psi, is maintained at this pressure for a portion of the injection stroke by allowing fuel to escape from the timing plunger chamber through a passage leading to a control valve. At a predetermined crank shaft angle occurring during the injection stroke, identified by well established methods, a control solenoid causes the control valve to close. Pressure is then increased in the timing plunger chamber and the control valve passage as a result of fuel compression by the downward motion of the timing plunger. This pressure creates a force acting upon a metering plunger, which then acts upon a closed metering chamber pre-filled with an appropriate volume of fuel. Thus, the pressure of the fuel charge already metered into the metering plunger chamber of the injector is increased. The pressure increase caused by the metering plunger, at a predetermined pressure, causes the injector nozzle to open and fuel then exits the injector. The injection or down stroke action of the injector coupling within the injector body insulates the timing and metering plunger from any undesirable side loads that may be transmitted from the valve train of the engine.
The upward or metering stroke of the injector coupling and accordingly the timing plunger is generally accomplished by the use of the return spring force acting on the injector coupling. The attachment of the timing plunger to the injector coupling is usually accomplished by a "T slot" arrangement, however, any method of physically joining the coupling to the timing plunger can be used. In such a physical connection arrangement, the top of the timing plunger is formed with a wide head sitting atop a narrow neck, and the bottom of the injector coupling is formed with a compatible receiving cavity to the top of the timing plunger. Thus, when the injector coupling is urged upward by the force of the return spring at the speed allowed by the withdrawal of the link and camshaft assembly, the timing plunger is drawn upward. The "T slot" interface transmits force from the coupling member to the timing plunger in both axial directions along the central axis of the injector body. A fresh fuel charge is then sequentially allowed to flow into the metering and timing plunger chamber by the control valve, to await the next engine cycle.
Several drawbacks exist in the fuel injector using the physically connected coupling/timing plunger combination described above. First, if the timing plunger for any reason binds or seizes within the bore of the injector body, the injector coupling becomes immobilized. In this situation, since the injector coupling, through the coupling return spring, also provides a restoring force to the link to ensure constant link contact with the camshaft, the link also becomes immobilized. Thus, a gap occurs between the camshaft and link interface. This gap, under some circumstances, causes the valve train to become imbalanced and vibrate at unpredictable and undesirable amplitudes and frequencies, especially at high engine speed and low load operating conditions, as the push rod is no longer balancing the loads on the valve train. Even more seriously, this gap can dislodge the push rod and allow the push rod to become a detached body within the valve train and cylinder head assembly, possibly resulting in significant and irreparable cylinder head and engine block damage.
Second, the "T-slot" configuration requires very close tolerances to ensure proper fit and function. The use of close tolerances is costly due to the complicated machining required. If the coupling/timing plunger interface or the "T slot" interface is out of tolerance, then timing plunger scuffing and seizing occurs, due to unpredictable and undesirable side loading.
Finally, the timing plunger's upward motion, due to the high change rate of the camshaft profile ultimately acting upon the push rod and mechanically attached coupling member can be extremely rapid at high engine RPM. This rapid motion can exceed the ability of the fuel injection supply device to provide fuel to the metering plunger chamber. Should this occur, cavitation results causing pressure fluctuation during the injection event and errors in the fuel mass injected.